Hundreds of thousands of protesters calling for President George Bush to be removed descended on to the streets of Manhattan yesterday, on the eve of the Republican party convention.

But as the demonstrators marched, Republican delegates arrived in town hoping to open a significant lead over the Democratic challenger, John Kerry, for the first time this year. While Mr Bush continued to campaign through the swing states at the weekend ahead of the convention, the race remained a virtual dead heat. A Time magazine poll gave Mr Bush a two percentage point lead, but that was less than the margin of error.

The march, of an estimated 250,000 people, passed off peacefully, despite predictions of violence, with demonstrators wearing badges stating "Re-defeat President Bush" and banners proclaiming "Bush lied, thousands died". Several dozen were arrested, including around 50 on bicycles, but the overall feeling was more carnival than carnage.

"He shouldn't be president and he shouldn't be coming to New York city to take advantage of 9/11," said Sarah Johnson, from Newark, New Jersey. "He's already making the rest of the world hate us and if he gets back in there's no saying what he might do."

The protest, flanked by police in riot gear, took place amid the tightest security in the history of US politics, with one security officer for every 2.4 delegates at the Republican convention.

"Today we send our message," said Leslie Cagan, the leader of United for Peace and Justice, which organised the largest of what promises to be a week of demonstrations. "We come from all walks of life ... from cities and towns across this nation and together we will march and in a resounding clear voice we will say no to the Bush agenda."

Before the rally speakers, including the filmmaker Michael Moore, and Jesse Jackson, the civil rights activist, called for an end to the occupation of Iraq and the Bush administration.

Kelly Doherty, a former military police sergeant who served in Iraq for a year and helped establish Iraq Veterans Against the War, said: "This is also dehumanising United States troops who are also having their sense of patriotism and loyalty perverted and used by an administration that would send our women and men to fight, die and kill for lies."

Mr Bush raised eyebrows in his latest defence of the Iraq war, referring in an interview to the "catastrophic success" of the initial invasion, which he said created problems further down the line.

"Had we to do it over again, we would look at the consequences of catastrophic success, being so successful so fast that an enemy that should have surrendered or been done in escaped and lived to fight another day," he said.

The convention will see the administration veer towards the centre in an attempt to win undecided voters and secure the loyalty of moderate Republicans. The main primetime speakers, apart from the president, will be moderates, and the keynote address will be delivered on Wednesday by a rightwing Democrat, Zell Miller, a senator from Georgia.

Campaigning in Ohio and West Virginia at the weekend, Mr Bush emphasised themes aimed at swing voters, such as education and health services.

He also took another step in distancing himself from the most heated issue of the summer, the assault on Senator Kerry's Vietnam war record by a group calling itself Swift Boat Veterans for Truth. In an interview on NBC television, he drew a distinction with his own, relatively safe, service in the Texas Air National Guard.

"I think him going to Vietnam was more heroic than my flying fighter jets. He was in harm's way and I wasn't," Mr Bush said.

However, his wife, Laura, told Time magazine that advertisements questioning Mr Kerry's honesty about his record were not unfair.

· Liam Fox, the Conservative party chairman, will fly into New York for the Republican convention today despite a row between Michael Howard and George Bush.